Color reproduction processes typically involve duplicating a color image from one medium to another medium, e.g., from one printed copy to another or from a display screen to a printed copy. Processes of this type are used in various application environments, including, for example, color proofing applications. In color reproduction processes, it is desirable to produce a duplicate whose coloration is highly similar to that of the original.
Accurate color rendering has been difficult to achieve, however. Display devices, such as printers, dye diffusion devices, slide printers, CRTs, and other electronic-type displays, have color rendering characteristics that are generally difficult to identify or difficult to model analytically, even after identification.
These characteristics are particular to classes or types of color display devices, as well as to each individual display device within a class. One such class, referred to as multi-color halftone printing devices, for example, produces a range of colors by using arrays of dots of a small number of colorants, or inks. Many printers of this type use what is known as a CMYK colorant scheme, which includes four color separations and four corresponding inks: cyan, magenta, yellow, and black.
Halftone printing devices use arrays of dots to simulate lighter tints as well as mixes of colors. In a typical multi-color, halftone printing technique, an original image is scanned through color filters to form a set of continuous-tone color separations. Each of the color separations represents intensities of one of the separated colors, such as yellow, at a plurality of pixel locations within the original image. The continuous-tone color separations are processed using a halftone screening system to produce a set of halftone color separations in the form of bitmaps. Each of the color separation bitmaps represents the bi-level condition of one of the separated colors at an addressable unit of the medium. For example, a typical four-color printing process uses four bitmaps, with each addressable unit of the medium having four associated bi-level conditions. The addressability of the color separation bitmaps ordinarily is much higher than the addressability of the continuous-tone color separations because several bi-level, addressable units are used to represent the intensity at a single continuous-tone pixel location.
In some applications, color separation bitmaps of this type are used to form halftone printing plates or to control a halftone printing mechanism such as a thermal mass-transfer device. In either case, the addressable units defined by the color separation bitmaps are imaged on a printing substrate by formation of dots carrying colorants that correspond to the separated colors. The dots are typically sized somewhat larger than the addressable units in order to provide a degree of partial overlap that prevents the appearance of gaps between adjacent dots in areas of solid color.
As addressed above, each halftone printer exhibits individual characteristics. These characteristics depend on, for example, the particular inks and paper used. Furthermore, the colorant channels in each individual halftone printer interact in a manner particular to the individual printer.
Several mathematical models have been proposed to characterize multi-color halftone printers. Many of these models are essentially analytic. For example, one model uses simple one-dimensional transfer functions to linearize CMYK values with respect to dot areas. Such analytic models typically use a relatively small number of parameters to model the behavior of devices and are primarily implemented using mathematical equations. Analytic models of this type, for many color reproduction applications, fail to consider sufficiently the individual printer characteristics discussed above.
Another class of color display devices, referred to as dye diffusion devices, uses a print head to produce color images. A dye-impregnated ribbon is typically laid over the sheet of paper on which the color image is to be printed. The print head impregnates the sheet with a certain concentration of the dyes contained in the ribbon. After the first color is printed, another ribbon impregnated with a dye of a different color is laid over the sheet. This process is typically repeated until all the colors have been printed. In a typical four-dye diffusion process, CMYK dyes are used.
Dye diffusion devices, like halftone printing devices, are also affected by various phenomena. For instance, a phenomenon known as dye inhibition occurs because subsequent dyes diffuse into the sheet of paper with more difficulty than initial dyes. If the cyan dye is diffused after the yellow dye, for example, the paper absorbs the cyan dye less readily than the yellow dye. Another phenomenon exhibited by typical dye diffusion processes is known as back transfer. Some of the dye diffused to the paper diffuses back into the ribbon. Each individual dye diffusion device exhibits individual characteristics that depend on, for example, the ribbon and paper used. These phenomena give rise to channel interactions that are difficult to model analytically due to their complexity.
Some types of analytical models proposed for characterizing dye diffusion printing processes use multiple-linear regression of channel-independent concentration estimates to correct for channel interactions. Such approaches, however, rely too heavily on regression equations. Due to the difficulty of accurately characterizing channel interactions using regression equations, these approaches often produce unacceptably large maximum color errors. Moreover, it is difficult to control the accuracy of these approaches at certain critical locations in color space, such as the primaries, secondaries, and neutrals.
Still another class of color display devices, known as cathode ray tubes (CRTs), produces colors by using an electron beam to illuminate phosphors of a small number of primary colors at pixels on a display screen. Most CRTs follow an RGB scheme, which uses red, green, and blue phosphors. When the electron beam illuminates closely-spaced phosphors, color blending creates the appearance of the color formed by combining the primary colors. For example, illuminating proximate red and green phosphors results in varying shades of yellow, depending on the intensity with which each phosphor is illuminated.
Several analytic models have been proposed for characterizing CRTs. Such models often use a gain-offset-gamma equation for each channel and a matrix for converting device-dependent (e.g., RGB) to device-independent color coordinates, such as Commission Internationale de L'Eclairage (CIE) XYZ tristimulus values. Models of this type yield reasonably accurate calorimetric characterizations for high quality CRTs that exhibit a high degree of independence between color channels. Like other types of display devices, however, CRTs typically exhibit channel interactions that are not adequately considered in these analytic models. Regression models have been proposed to account for these channel interactions. As with channel interactions affecting dye diffusion devices, however, these channel interactions are difficult to characterize using regression models. As a result, using regression models to account for channel interactions often results in undesirable and sometimes unacceptable maximum errors. In addition, some lower-quality CRTs exhibit other idiosyncrasies, such as changes in channel chromaticity with variations in digital color values. Behavioral characteristics of this type cause the response of such lower-quality CRTs to deviate from analytic models.